Transformation of Streptococcus sanguis Challis with a plasmid of Streptococcus pneumoniae

The present work is concerned with plasmid transformation of Streptococcus sanguis strain Challis with derivatives of pDP1/pSMB1, the only plasmid found to occur naturally in Streptococcus pneumoniae. Two recombinant plasmids derived from the cryptic pSMB1 were used: pDP27 (4.5 kb) conferring resistance to chloramphenicol (Cm), and pDP28 (7.8 kb), a shuttle plasmid, conferring resistance to Cm in Escherichia coli, and resistance to erythromycin (Em) in pneumococcus. It could be shown that pSMB1 can replicate in S. sanguis; in fact, Challis strain V288 was transformed to Cm-resistance and to Em-resistance by pDP27 and pDP28 respectively. Shuttle plasmid pDP28 can transform S. sanguis both when isolated from pneumococcus and from E. coli, albeit with a different efficiency. The low frequency of transformation observed when pDP28 was isolated from E. coli DH1 (recA) was shown to be due to lack of multimeric forms of the plasmid in the DNA preparations obtained from this strain. When pDP28 was isolated from E. coli C600 (RecA+), multimeric forms were present, and transformations of S. sanguis was more efficiency Using pDP28 as vector in cloning experiments, where S. sanguis was the host of the recombinant DNA molecules, treatment of the vector with alkaline phosphatase inhibited the recovery of recombinant clones.